Introduction and Historical statement.
A careful examination of the literature on the atomic weight of cadmium will convince any one that considerable uncertainty yet remains in reference to this constant. Six experimenters have worked on this problem but the results of no one of them can be accepted as being more accurate than those of all others. The value assigned to cadmium varies from 111.48 to 112.32 on the basis of oxygen = 16. The best work has apparently been done by von Hauer, Lenssen and Huntington. The results of these three seem entitled to about equal confidence, yet the figure obtained by von Hauer differs from that of Huntington by three tenths of a unit.
The more prominent difficulties which have been encountered were:
First. The preparation of cadmium compounds free from all impurities, and which at the same time were well adapted to weighing.
Second. The lack of a thoroughly simple and exact method for the analysis of cadmium compounds.
Third. Insufficient care in weighing in many cases whereby small errors were introduced into the results.
The methods which have been employed are:
1 Conversion of the metal into the oxide. (Stromeyer).
2 Conversion of the sulphate into the sulphide. (von Hauer and Partridge).
3 Decomposition of the oxalate to the oxide. (Lenssen and Partridge).
4 Determination of the chlorine in cadmium chloride, by which the relation between the chloride and metallic silver was established. (Dumas.)
5 Precipitation of the bromine in cadmium bromide as silver bromide. (Huntington.)
6 The conversion of the oxalate into the sulphide. (Partridge.)
The different pieces of work will be taken up in chronological order and briefly considered.
Stromeyer, Schurigg Journ. 22, 366. 1818, determined the atomic weight of cadmium a short time after the discovery of the element. He does not describe his method in detail but established the relation between cadmium and oxygen to be:
Cd : O = 100 : 14.352.
| If the atomic weight of | oxygen = | 16, |
| ” ””” | cadmium = | 111.483. |
The very low result as compared with all subsequent work was probably due to the presence of a small amount of zinc, since the cadmium used was obtained from zinc ores and no adequate means of separation from the zinc is described.
von Hauer, Journ. f. prakt. Chem. 72, 338. 1857. His method consisted in reducing a weighed amount of cadmium sulphate to the sulphide in a stream of hydrogen sulphide, under pressure, at an elevated temperature, and weighing the sulphide. The reduction was shown to be complete by proving the absence of sulphate in the sulphide.
| 64.2051 | grams of | cadmium sulphate | |
| gave | 44.4491 | ” ” | ” sulphide. |
| If the atomic weight of | oxygen = | 16, |
| ” ” ””” | sulphur = | 32.059, |
| ” ””” | cadmium = | 111.935. |
The atomic weight of cadmium calculated as an average of the nine determinations made using the above values for oxygen and sulphur = 111.94.
- Maximum, 112.121.
- Minimum, 111.796.
- Mean, 111.940.
The work of von Hauer is greatly to be preferred to that of Stromeyer. The large amount of material used in each determination tended to lessen any experimental error. A very considerable degree of care seems to have been exercised in purifying the cadmium sulphate. In determinations 1-5 a different specimen of sulphate was employed from that in determinations 6-9. The average value found in the first five determinations = 111.910, in the last four = 111.977. The close agreement between the results obtained from the different preparations of the sulphate argues in favor of a fair degree of purity for all the material.
The method of weighing the more or less hygroscopic cadmium sulphate is open to criticism when employed in accurate work. The cadmium sulphate was placed in an open boat, dried, cooled over sulphuric acid, and weighed. It was again dried, cooled as before, and weighed. The second weighing could be quickly accomplished since the approximate weight was known. The two weighings agreed to within less than a milligram or a third drying and weighing were made. An error of a milligram in the weight of the sulphate produced an average error in the atomic weight of cadmium of about .06. That a discrepancy of greater or less magnitude was introduced from this source will be readily seen.
Dumas Ann. Chim. Phys. 55, 158. 1859, determined the relation between cadmium chloride and the metallic silver required to precipitate the chlorine. Metallic cadmium was dissolved in boiling hydrochloric acid and the solution evaporated. The cadmium chloride was fused for five or six hours in a stream of hydrochloric acid gas. Six determinations were made. 23.0645 grams of cadmium chloride were equivalent to 27.173 grams of metallic silver.
| If the atomic weight of | silver = | 107.93. |
| ” ” ””” | chlorine = | 35.45. |
| ” ””” | cadmium = | 112.322. |
The atomic weight of cadmium calculated as the average of the six determinations made, using the above values for silver and chlorine = 112.241.
- Maximum, 112.759.
- Minimum, 111.756.
- Mean, 112.241.
The large difference between the results would indicate some considerable source of error in part or all of the determinations. The first three determinations were made from a different specimen of cadmium from the last three.
In the first three the cadmium used does not seem to have been purified and the cadmium chloride prepared from it was more or less tinted brown. In the last three a new specimen of metal was used which in Dumas’ words could reasonably be considered to be absolutely pure. The chloride prepared from it was colorless, well crystallized and perfectly soluble in water. In order to show clearly the wide discrepancy between the results obtained from the two specimens of cadmium which were used, the separate determinations are given in detail.
| At. Wt. | |||
|---|---|---|---|
| CdCl₂ | Ag. | Cadmium. | |
| 1 | 2.369 | 2.791 | 112.322 |
| 2 | 4.540 | 5.348 | 112.347 |
| 3 | 6.177 | 7.260 | 112.759 |
| 4 | 2.404 | 2.841 | 111.756 |
| 5 | 3.5325 | 4.166 | 112.135 |
| 6 | 4.042 | 4.767 | 112.130 |
The average result of the first three determinations = 112.476. The average result of the last three determinations = 112.007. From Dumas’ own statement concerning the purity of the cadmium chloride analyzed, determinations 4-6 are much to be preferred to determinations 1-3 and the most probable value from Dumas’ work would be very nearly 112.
Lenssen Journ. f. prakt. Chem. 79, 281. 1860, regarded the oxalate of cadmium as well adapted to the determination of the atomic weight of cadmium. A solution of cadmium chloride which had been purified by repeated crystallization was treated with an excess of a solution of pure oxalic acid. The cadmium oxalate formed was filtered off, washed, and carefully dried in the air at 150° C. until the last trace of water was removed. 1.5697 grams cadmium oxalate gave 1.0047 grams cadmium oxide.
| If the atomic weight of | oxygen = | 16, |
| ” ” ”” | carbon = | 12.003, |
| ” ”” | cadmium = | 112.043. |
The average of the three determinations using the above values for oxygen and carbon is 112.067.
- Maximum, 112.304.
- Minimum, 111.911.
- Mean, 112.067.
The small amount of material used in each determination, the small number of determinations made, and the rather large difference between the highest and lowest result are objectionable. There are certain weak points in the method but to these reference will be made later.
Huntington, Proc. Amer. Acad. 17, 28. 1882, working with Cooke, made two series of determinations of the atomic weight of cadmium. In the first series the relation between cadmium bromide and the silver bromide formed from it was determined. In the second, the relation between cadmium bromide and the silver required to precipitate the bromine.
The cadmium bromide was prepared by dissolving the carbonate in hydrobromic acid and subliming the product in a stream of carbon dioxide.
In the first series of eight determinations 23.3275 grams of cadmium bromide were equivalent to 32.2098 grams of silver bromide.
| If the atomic weight of | silver = | 107.93. |
| ” ” ””” | bromine = | 79.95. |
| ” ””” | cadmium = | 122.239. |
- Maximum, 112.290.
- Minimum, 112.169.
Where the difference between the maximum and minimum value is slight, the average of the separate determinations agrees closely with the number found by comparing the total substance used with the total product obtained. The latter method of calculation seems however to be preferable.
In the second series of eight determinations 28.6668 grams of cadmium bromide were equivalent to 22.7379 grams of silver.
Using the same values for silver and bromine, the atomic weight of cadmium = 112.245.
- Maximum, 112.320.
- Minimum, 112.180.
The agreement of the separate determinations with each other is fairly close and the average of the two series of determinations is nearly the same. Huntington took great care in the purification of his material and in the carrying out of his method, which are strong arguments in favor of his work, yet his method is not as simple as could be desired where the nature of the work demands the greatest possible accuracy in all details and it also appears to be subject to some of the errors common to ordinary analytical operations.
Partridge. Amer. Journ. Science XL, 377. 1890. Methods: 1ˢᵗ. Decomposition of the oxalate to the oxide. 2ⁿᵈ. Reduction of the sulphate to the sulphide. 3ʳᵈ. Conversion of the oxalate into the sulphide. As an average of the determinations made by each method Partridge gives:
| 1ˢᵗ series, | atomic weight of cadmium | = 111.8027. |
| 2ⁿᵈ ” | ”” ”” | = 111.7969. |
| 3ʳᵈ ” | ”” ”” | = 111.8050. |
An excellent agreement between results obtained by different methods[1].
That this very close agreement is only apparent has been shown by Clarke. He has found that the above calculations are based on the assumption that the atomic weight of carbon = 12, and that of sulphur = 32 when oxygen = 16. There seems to be little justification for this rather arbitrary selection by Partridge since the most refined work shows that whole numbers do not express the most probable atomic weights of carbon and sulphur in a system where oxygen = 16.
The atomic weight of cadmium calculated from the total material used and the total product found in each of the three series is:
| O = 16. | C = 12. | S = 32. | At.Wt.Cd. | |
|---|---|---|---|---|
| 1ˢᵗ series, | CdC₂O₄ : CdO = | 12.66368g. : | 8.10031g. | 111.805. |
| 2ⁿᵈ ” | CdSO₄ : CdS = | 15.93505g. : | 11.02691g. | 111.786. |
| 3ʳᵈ ” | CdC₂O₄ : CdS = | 16.85228g. : | 12.12906g. | 111.806. |
| difference, | 0.020. | |||
| O = 16. | C = 12.003 | S = 32.059 | At.Wt.Cd. | |
|---|---|---|---|---|
| 1ˢᵗ series, | CdC₂O₄ : CdO = | 12.66368g. : | 8.10031g. | 111.816. |
| 2ⁿᵈ ” | CdSO₄ : CdS = | 15.93505g. : | 11.02691g. | 111.727. |
| 3ʳᵈ ” | CdC₂O₄ : CdS = | 16.85228g. : | 12.12906g. | 111.610. |
| difference, | 0.206. | |||
As Clarke has pointed out when those values are chosen for carbon and sulphur which are founded on the best experimental evidence the agreement between the different series of results as calculated by Partridge is somewhat modified.
I have repeated the work on which series I is based and would call attention to the following points in which it appears to have been experimentally defective.
1 The metal was only distilled twice in a vacuum. It has been found in this laboratory that perfectly pure cadmium or zinc can be prepared only by repeated distillations, each one being carried on slowly to allow the impurities to separate by means of their difference in volatility.
2 The supposed mixture of metal and oxide resulting from the decomposition of the oxalate was only moistened with a few drops of nitric acid in order to reoxidize any reduced metal. Unless the entire mass of metal and oxide was dissolved there would be danger of the presence of free undissolved metal which would possess an appreciable vapor-tension below the temperature of decomposition of cadmium nitrate. An appreciable loss in weight resulting from a distillation of the metal out of the crucible might easily result.
3 It seems very probable that the cadmium nitrate was not heated sufficiently to remove all traces of the oxides of nitrogen. I have found that this could only be accomplished by long continued heating. Constant weight was not sufficient to have decided this point since it was also found that this could be reached short of complete decomposition, if the temperature was too low to remove the last traces of these oxides. Some very delicate test for such oxides should have been applied at the end of each experiment.
The following table contains a summary of the results thus far obtained.
When two values are given for one series of determinations, the first is calculated from the total material used and the total product found, the second is an average of the results of the separate experiments. Oxygen is taken as 16 throughout.
| Date. | Investigators. | At.Wt.Cd. | ||
|---|---|---|---|---|
| 1818, | Stromeyer, | 111.483 | ||
| 1857, | von Hauer, | 111.935 |  | |
| 111.940 | ||||
| 1859, | Dumas, | 112.322 | | |
| 112.241 | ||||
| 1860, | Lenssen, | 112.043 | | |
| 112.067 | ||||
| 1882, | Huntington, | 1ˢᵗ series | 112.239 | |
| ” | 2ⁿᵈ ” | 112.245 | ||
| 1890, | Partridge, | 1ˢᵗ series | 111.805 | |
| ” | 2ⁿᵈ ” | 111.786 | ||
| ” | 3ʳᵈ ” | 111.806 | ||
In the above calculation of Partridge’s results C = 12. S = 32. In the following carbon is taken as 12.003 and sulphur is 32.059.
| 1890, | Partridge, | 1ˢᵗ series | 111.816 | |
| ” | 2ⁿᵈ ” | 111.727 | ||
| ” | 3ʳᵈ ” | 111.610 |
After a careful examination of the methods available it becomes evident that no one of them was per se as accurate as the method employed by Morse and Burton,[2] for the determination of the atomic weight of zinc, and more recently by Burton and Morse,[3] for the determination of the atomic weight of magnesium. The method of work was to prepare pure metallic cadmium, to convert a weighed portion of the metal into nitrate by means of pure nitric acid, to decompose the nitrate completely to oxide and to weigh the oxide.